1. Field of Invention
The present invention relates to a method for patterning a material layer. More particularly, the present invention relates to a double patterning technology for forming a random opening pattern in a material layer.
2. Description of Related Art
Due to the great demand of higher and higher integration, integrated circuit devices have to be fabricated with a smaller and smaller dimension. The photolithography process is a very crucial step that affects the dimension and performance of a semiconductor device. For example, in a metal-oxide semiconductor (MOS) device, the pattern of various thin films and the dopant regions are all determined by this photolithography step. Currently, device integration has reached a linewidth of 0.06 micron. The development of the photolithography process thus determines whether the linewidth can be approached. As a result, methods such as optical proximity correction (OPC) and phase shift mask (PSM) have been proposed and used.
Optical proximity correction is used to eliminate the deviation of critical dimension caused by the proximity effect. When a light beam penetrates through a photomask to transfer the pattern on the photomask to a wafer, the light beam incident on the wafer is diffused due to scattering. On the other hand, the light beam is reflected from the surface of the wafer to cause interference of the incident light and consequently cause a double exposure to change the actual volume of exposure.
When a pattern for being transferred onto a target material has both of the dense sub-pattern and the iso-sub-pattern, it is necessary to design various dummy patterns to be disposed aside the dense sub-pattern and the iso-sub-pattern to eliminate the exposure environment difference between the dense sub-pattern and the iso-sub-pattern. Besides, the proximity effect is high during the pattern transferring process of the pattern having has both of the dense sub-pattern and the iso-sub-pattern. Accordingly, the size of the sub-patterns is restricted by the optical limit of photolithography technology. Also, the process window for transferring such pattern is small.